The emergence of antibiotic-resistance among bacterial pathogens is becoming a serious and growing threat to human health and welfare. This resistance is due, in part, to the recently discovered multiple drug resistant (MDR) efflux pumps, which can handle a wide range of structurally different compounds. Moreover, these NIDR systems can be amplified in resistant cells and can shift or expand their substrate profiles with mutation making them a major threat to drug therapy. Today we face the frightening possibility that many, if not all pathogenic bacteria may soon become resistant to all known antibiotics. Hence, understanding the structures, functions and mechanism of action of these efflux pumps will be important in the design of new drugs which can inactivate or circumvent the action of these MDR pumps. The MDR efflux pump in Gram-negative bacteria is composed of three different protein species, a cytoplasmic membrane transporter protein, a periplasmic fusion protein, and an outer membrane efflux protein. During the functioning of this pump these proteins form a complex, which translocates its substrate across the two membranes of the cell envelope and into the surrounding environment. TolC, which has relatives in most Gram-negative bacteria, is a broad-based outer membrane efflux protein, which interacts with a variety of protein export pumps and MDR efflux pumps of Escherichia coli. Understanding TolCs' structure-function relationship with such pumps should provide novel and important insights into these bacterial NIDR systems. To ascertain such information we propose to utilize a large collection of TolC mutants to map the structure-function interactions of TolC with a specific MDR efflux pump, VceAB. These studies will entail physiological, biochemical and genetic analysis of the ToIC-VceAB interactions, which may eventually lead to the design of novel and effective anti-microbial agents.